Recovering oil from an oil containing reservoir can include three distinct phases. During a first phase, natural pressure of the oil containing reservoir and/or gravity can drive oil into a wellbore, and combined with an artificial lift technique, such as pumping, bring the oil to the surface. However for some oil containing reservoirs, in the first phase only about 10 percent of the original oil in place is recovered.
A second phase, to extend the productive life of the oil containing reservoir, can increase oil recovery to 20 to 40 percent of the original oil in place. For some applications, the second phase can include injecting water to displace oil and drive it to a production wellbore. In some applications, reinjection of natural gas has been employed to maintain and/or increase reservoir pressure, as natural gas is often produced simultaneously with the oil recovery.
However, with much of the easy-to-recover oil already recovered via the first phase and/or the second phase, a third distinct phase of oil recovery has been developed. The third phase may be referred to as enhanced oil recovery. Enhanced oil recovery techniques offer prospects for producing more of the oil containing reservoir's original oil in place, thus further extending the productive life the oil containing reservoir. Worldwide, one estimate of oil in place that is not recoverable by the first phase of oil recovery or the second phase of oil recovery that could be the targeted by enhanced oil recovery techniques is 377 billion barrels of oil. Enhanced oil recovery can include an injection of fluids other than water, such as steam, gas, alkali, surfactant solutions, various polymers or carbon dioxide (CO2).
For some applications the fluid is miscible with the hydrocarbons in the oil containing reservoir. This fluid injection can help reduce the viscosity of oil present in the oil containing reservoir in order to increase the flow of oil to the production wellbore.
Miscible carbon dioxide injection, however, can be accompanied with a number of drawbacks. One problem encountered is poor sweep of the oil containing reservoir. Poor sweep can occur when carbon dioxide injected into the oil containing reservoir flows through the paths of least resistance (i.e. more permeable zones) due to the low viscosity of the carbon dioxide, thus bypassing significant portions of the oil containing reservoir and the oil located there. In addition, due to the low density of the carbon dioxide, the injected carbon dioxide can rise to the top of the formation and “override” portions of the formation, leading to early breakthrough of the carbon dioxide at the production wellbore, leaving less carbon dioxide within the oil containing reservoir to contact with the oil.
To increase the enhanced oil recovery process effectiveness, a surfactant has been used to generate an emulsion in the formation. An emulsion can generate an apparent viscosity of about 100 to about 1,000 times that of the injected carbon dioxide, therefore, the emulsion can inhibit or slow the flow of the carbon dioxide into the path of least resistance. In other words, the emulsion can serve to block the volumes of the oil containing reservoir through which the carbon dioxide can short-cut, thereby reducing its tendency to channel through highly permeable fissures, cracks, or strata, and direct the carbon dioxide toward previously unswept portions of the oil containing reservoir. As such, the emulsion can help force the carbon dioxide to the recoverable hydrocarbons in the less depleted portions of the oil containing reservoir.